The subject invention pertains to blends of polymers, to a process for preparation thereof, and to products fabricated from such blends.
Elastomers are defined as materials which experience large reversible deformations under relatively low stress. Elastomers are typically characterized as having structural irregularities, non-polar structures, or flexible units in the polymer chain. Some examples of commercially available elastomers include natural rubber, ethylene/propylene (EPM) copolymers, ethylene/propylene/diene (EPDM) copolymers, styrene/butadiene copolymers, chlorinated polyethylene, and silicone rubber. Preferably, an elastomeric polymer can be stretched to at least twice its relaxed length with stress and after release of the stress returns to approximately the original dimensions and shape. Preferably elastic recovery of an elastomer before crosslinking as measured by ASTM D-412 is at least about 40 percent, more preferably at least about 60 percent, most preferably at least about 80 percent.
Thermoplastic elastomers are elastomers having thermoplastic properties. That is, thermoplastic elastomers are optionally molded or otherwise shaped and reprocessed at temperatures above their melting or softening point. One example of thermoplastic elastomers is styrene-butadiene-styrene (SBS) block copolymer. SBS block copolymers exhibit a two phase morphology consisting of glassy polystyrene domains connected by rubbery butadiene segments. At temperatures between the glass transition temperatures of the butadiene midblock and the styrene endblocks, that is, at temperatures from -90.degree. C. to 116.degree. C., the SBS copolymers act like a crosslinked elastomer.
In contrast, thermoset elastomers are elastomers having thermoset properties. That is, thermoset elastomers irreversibly solidify or "set" when heated, generally due to an irreversible crosslinking reaction. In the practice of the instant invention, a gel content of at least about 20 weight percent based on total elastomer as measured by xylene extraction is considered thermoset. Two examples of thermoset elastomers are crosslinked ethylene-propylene monomer rubber (EPM) and crosslinked ethylene-propylene-diene monomer rubber (EPDM). EPM materials are made by copolymerization of ethylene and propylene. EPM materials are typically cured with peroxides to give rise to crosslinking, and thereby induce thermoset properties. EPDM materials are linear interpolymers of ethylene, propylene, and a nonconjugated diene such as 1,4-hexadiene, dicyclopentadiene, or ethylidene norbornene. EPDM materials are typically vulcanized with sulfur to induce thermoset properties, although they alternatively are optionally cured with peroxides. While EPM and EPDM materials are advantageous in that they have applicability in higher temperature applications, EPM and EPDM elastomers suffer the disadvantages of low green strength (at lower ethylene contents), of a higher susceptibility of the cured elastomer to attack by oils than characteristic of styrene butadiene rubbers, and of resistance of the cured elastomer to surface modification.
Thermoplastic vulcanizates (TPV's) are polyolefinic matrices, preferably crystalline, through which thermoset elastomers are generally uniformly distributed. Examples of thermoplastic vulcanizates include EPM and EPDM thermoset materials distributed in a crystalline polypropylene matrix. One example of a commercial TPV is Satoprene.TM. thermoplastic rubber which is manufactured by Advanced Elastomer Systems and is a mixture of crosslinked EPDM particles in a crystalline polypropylene matrix. These materials have found utility in many applications which previously used vulcanized rubber, e.g. hose, gaskets, and the like TPV's are noted for their processability as thermoplastics while retaining the excellent tensile and compression set properties of vulcanized rubbers.
Commercial TPV's are typically based on vulcanized rubbers in which a phenolic resin or sulfur cure system is used to vulcanize, that is to crosslink, a diene (or more generally, a polyene) copolymer rubber by way of dynamic vulcanization, that is crosslinking while mixing (typically vigorously), in a thermoplastic matrix. Sulfur or a phenolic resin is preferred over peroxide free radical cure systems because peroxide degrades a polypropylene or and crosslinks a polyethylene as well as the rubber and this is in turn limits the extent of rubber crosslinking that can occur before the entire mixture degraded or crosslinked and is no longer thermoplastic.
In the art such as evidenced by PCT patent application WO96/07681 (McKay et al.) the preferred method of preparing a thermoplastic vulcanizate is to form an admixture of non-crosslinked elastomeric polymer and polyolefin resin and curing agent then masticate the admixture at a vulcanization temperature. Preferably the non-crosslinked polymer, polyolefin are intimately mixed before a curing agent is added.
C--H insertion reagents like poly(sulfonyl azide)s have been used to achieve coupling of certain elastomers and blends for instance as disclosed in copending U.S. applications: U.S. application Ser. No. 60/057,713, filed Aug. 27, 1997; U.S. application Ser. No. 09/140,900, filed Aug. 26, 1998 and U.S. application Ser. No. 09/129,163 filed Aug. 5, 1998 to achieve coupling which involves no more than about 10 percent, preferably less than about 2 weight percent gel, depending on the polymer and purpose for which it is used. Using advantageously less than about 0.3 weight percent poly(sulfonyl azide) based on polymer results in retaining thermoplastic properties. In a blend, the coupling processes do not result in crosslinking an elastomeric phase. It would be desirable to have a process for making a thermoplastic vulcanizate using a C--H insertion curing agent that can be compression molded, advantageously having a compression set at 70.degree. C. less than that of a coupled polymer as taught in the cited references, preferably less than 97 percent, more preferably less than about 90 percent, most preferably less than about 70 percent compression set.